BIOS interrupts, priviledge levels and paging
- by Jack
Hi, I was learning about Intel 8086-80486 CPUs and their interactions with HW. But I still don´t understand it quite well. Please, help me fill blank spots.
First, I know that CPU communicates with HW using BIOS interrupts. But, what really happens in PC, when I call some INT instruction? I know that according the interrupt table some instructions begin to execute, but how by executing some instructions can BIOS recognize what I want to do? Becouse as far as I know, CPU has no extra communication channel with BIOS, it can only adress memory and receive data. So how can I instruct BIOS to do something, when I can only adress RAM?
Next thing I dont understand is about priviledge levels. I know about ring model, and acess rights, but how CPU knows which priviledge level has executed instruction? I think that these priviledges apply only when intruction is trying to adress memory, but how applications gets its priviledge level? I mean I know its level 3, but how its set?
And last thing, I know that paging is adress scheme that is used to support aplication-transparent virtual memory, or swaping, but I could not find any informations about how is paging tied with protected mode. Like if paging is like next mode independent of protectet mode, or its somehow implemented within protected mode. And if it is implemented in protected mode, isn´t it too slow, to first adress application space, than offset, and than paging folder, page and offset once again? Thank you for every response.
